Audiometer



Sept. 113, 1949. w 1:. BLOK 2,481,973

AUDIOMETER Filed May 3, 1946' Patented Sept. 13, 1949 UNITED STATES PATENT OFFICE by mesne assignments, to Hartford National Bank and Trust Company, Hartford, (1onn., as

trustee Application May 3, 1946 Serial No. 665,911 In the Netherl nd ust 1 194.

Section 1, Public Law 690, August 3, 1946 Patent expires August 14, 19.61

4 Claims. 1

For the exact investigation of the auditory ac ne s of e s ns it s d si a the decline f t e sensitivity of ar r a v ly to tha of a no m l p son should be det i q t tively as a function o f quency.

F r; uch an inv s i ation u e is p f ly m de o a so-called audiome er by me ns of whi h n audible ton f ad ust ble frequ ncy n t l s intermittently adjustable intensity may be produced.

Audiometers are known in which the low-frequency alternating voltages of adjustable frecluency produced with the aid of a valve generator, after being amplified with the aid of one or more amplifying tubes, are supplied to a reproducing device via an adjustable attenuator.

As compared with the usual applications of low-frequency generators and amplifiers, in an audiometer it is much more necessary that attention should be paid to the fact that interfering noise such, for example, as the noise of the amplifier, mains hum, etc. does not occur since this might act upon the obtained test results in a manner which cannot be controlled objectively.

The present invention provides an improve.

ment of an audiometer whereby a parasitic noise which under certain conditions highly disturbs the test results is avoided in a simple manner.

As has been found by applicant, in an audiometer of the above-described type an interfering noise occurs if the amplifier comprises an output impedance having a laminated iron core, for example an output transformer. This parasitic noise is brought about by the mechanical forces,

exerted on the laminations, of the magnetic field produced by the amplified low-frequency alterhating voltage (soecalled singing of the transformer) and is particularly interfering if in connection With the ear sensitivity of the patient the signal taken from the output impedance of the amplifier must be considerably attenuated before being supplied to the reproducingdevice. In this case it occurs that the patient no longer perceives the sound originating from the reproducing device but perceives the parasitic noise originating dh'ectly from the output transformer, which leads to wrong test results.

Pla ngthe p t en sufiiclentl far wa fro the amplifier might have the efiect of suppressing this disturlsgarrce but this is undesirable for practical reasons.

According to the invention, the stated draw,- back is obviated by arranging at least part of the attenuator before the said output impedance.

When great attenuation factors are US$ 1 the normal alternating voltage at the output pedance of the amplifier no longer occurs but nly a ra ti n thereof which is det min d by the attenuator part which is arranged before the out.- put impedance, the stated drawb ck thus sufficiently avoided.

In view of errors in measurement which may be brought about by the noise of the amplifie V advantageous that the attenuator part Wqlfil is arranged before the said output impedance should he placed behind the amplifier, i. e, between the last amplifying tube and the output impedance.

As has been found by applicant, for obtaining reliable test results it is more advantageous, how.- ever, to arrange a part of the attenuator before the amplifier.

Although this measure renders the signal to noise ratio which exists at the output impedance of the amplifier less advantageous, it has been found that with normal proportioning of the amplifier a more serious source of faults is counteracted, viz, the errors in measurement brought about by the non-linear distortion produced in the amplifier.

The last-mentioned faults may occur in par-.- ticular with low frequencies, not only since with these frequencies the distortion of the usual low.- froquenoy amplifiers is usually greatest but in particular due to the characteristic variation of the curve of the normal ear sensitivity in this fre.- quency range. In measurements with the aid of a tone having a frequency of cycles/sec, the first harmonic of this frequency 0. s/sec. for e m l must b at le t by 18 dbs. weak t a the fun a en al t n n order at t e normal car may not hear the said overtone earlier than the fundamental tone. With diverging cu ve of th ar sensitiv t e difference in in n it ne ess ry to a i e o s i m m n may be even con id ra g ea so hat on exces iv l sm l d rt n in the amp ie mu occu Connecting a part of the attenuator before the amplifier input ha e ef ect o co r bly decreasing the load of the amplifier and hence the di ortion produc d whi h. as te b ha gee found advantageous in spite of the fact that 1 noi e of the a plifie u om ar v y increaseslt is mention d at with n ma p or onin f the amp fier i has b en fou s n g o s to conn t he whole t enua or before the amp ih r which in itsel is. simpl r an w u d e tter to id t e So=ca11ed win ing o the ou pu impedance and the distortion, since in this case measurements with low tone-intensity the noise of the amplifier becomes interfering.

The invention will be more clearly understood by reference to the accompanying drawing, given by way of example, which represents diagrammatically one highly advantageous form of construction of an audiometer according to the invention.

In the audiometer shown, the generator G which produces a low-frequency alternating voltage of adjustable frequency, is constituted for practical reasons by two differently tuned highfrequency oscillators O1 and 02 serving respectively for the coarse and the fine adjustments, a frequency changer M, and a filter F which serves to derive the desired audible difference frequency from the frequency changer, which difference frequency, after being amplified with the aid of an amplifier V including an output transformer T, is supplied to a loudspeaker L through a portion of an attenuator D.

For adjusting the intensity of the signal supplied to the loudspeaker, which is required for measuring, there is provided an adjustable attenuator D which permits of obtaining an attenuation adjustable, for example, in steps of dbs. and a maximum attenuation of 105 dbs. The attenuator comprises three parts, viz. two parts D1 and D2 which may either be switched-in at will or are not adjustable per se, hence fixed, each of which brings about an attenuation of 25 dbs. and a variable part D3 having 11 steps each of 5 dbs. The attenuator parts D2 and D3 are interposed between the output transformer T of the amplifier V and the loudspeaker L, whereas part D1 is interposed between the generator G and the input circuit of the amplifier V. If the required attenuation is smaller than 55 dbs., use is solely made of the attenuator part D3, the parts D1 and D2 being switched off.

If, however, a greater attenuation is required, the parts D1 and D2 are also used, the switches by means of which these last-mentioned parts may be switched-in being coupled, preferably mechanically, in such manner that at first the attenuator part Di interposed between the generator and the amplifier is switched, the part Dz being switched only if still greater attenuation factors are required. In measurements with the aid of a signal of low intensity a disturbing "singing of the output impedance, which may of course alternatively be constituted by a choke coil, is thus avoided with security, while with the stated proportioning of the attenuator parts disturbances due to the non-linear distortion and the noise of the amplifier are likewise suppressed.

What I claim is:

1. An audiometer comprisin a generator for producing alternating voltages within the audio frequency range, an amplifier coupled to said generator for amplifying said alternating voltages, said amplifier including an inductive output impedance having a laminated iron core, a reproducing device coupled to said impedance to derive therefrom the amplified alternating voltages, an adjustable attenuator for controlling the magnitude of the amplified alternating voltages supplied to said device, means connecting a portion of said attenuator between said generator and said amplifier to reduce the production of parasitic noises by said impedance, and means connecting the remaining portion of said attenuator between said amplifier and said device to control the magnitude of voltage supplied to said device.

2. An audiometer comprising a generator for producing alternating voltages within the audio frequency range, an amplifier coupled to said generator for amplifying said alternating voltages, said amplifier including an inductive output impedance having a laminated iron core, a reproducing device coupled to said impedance to derive therefrom the amplified alternating voltages, an adjustable attenuator for controlling the magnitude of the amplified alternating voltages supplied to said device and including a first portion which is adjustable within a predetermined range of values, a second portion of fixed value and a third portion of fixed value, means connecting said first portion of said attenuator between said amplifier and said device, means connecting said second portion between said amplifier and said first portion, and means connecting said third portion between said generator and said amplifier to reduce-the production of parasitic noises by said impedance.

3. An audiometer comprising a generator for producing alternating voltages within the audio frequency range, an amplifier coupled to said generator for amplifying said alternating voltages, said amplifier including an inductive output impedance having a laminated iron core, a reproducing device coupled to said impedance to derive therefrom the amplified alternating voltages, an attenuator for adjusting the magnitude of the amplified alternating voltages supplied to said device and including a first portion which is adjustable within a predetermined range of attenuation values, a second portion having a fixed value and a third portion having a fixed value, said first portion being interposed between said amplifier and said device, and switching means arranged in a first position to interpose said second portion between said amplifier and said first portion and in a second position to interpose said frequency range, an amplifier coupled to said generator for amplifying said alternating voltages, said amplifier including an output transformer having a laminated iron core, a reproducing device coupled to said output transformer to derive therefrom the amplified alternating voltages, an attenuator for adjusting the magnitude of the amplified alternating voltages supplied to said device and including a first portion interposed between said amplifier and said device, said first portion being adjustable within a predetermined range of attenuation values, a second portion having a fixed value not exceeding the maximum value of said first portion and a third portion having a fixed value equal to the value of said second portion, and switching means arranged in a first position to interpose said second portion between said amplifier and said first portion and in a second position to interpose said third portion between said generator and said amplifier. LOURENS BLOK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,750,960 Langenbeck et a1. Mar. 18, 1930 2,287,401 Wengel June 23, 1942 

